1. Field of the Invention
The present invention relates to a monochrometer for selecting only electrons having a certain energy spread from an electron beam having an energy distribution and for causing the selected electrons to hit a specimen and, more particularly, to a monochrometer that can be preferably used within an electron gun.
2. Description of the Related Art
In recent years, field emission guns (FEGs) have had wide acceptance as electron guns for electron microscopes and the like. An FEG is a source having high brightness and emits an electron beam of an energy spread of about 0.3 to 0.7 eV. With this energy spread, however, a desired energy resolution may not be obtained in some kinds of analyses.
For example, in electron energy loss spectroscopy (EELS) where an energy loss that electrons suffered within a specimen is measured to investigate the electron structure of the material, the energy resolution of a spectrometer used to measure the energy loss in the specimen is estimated to be less than 0.1 eV. Where the energy spread of electrons incident on the specimen is about 0.3 to 0.7 eV, the energy resolution for analysis is restricted by the energy spread of the electron beam incident on the specimen. Therefore, a finer energy spectrum cannot be observed.
Accordingly, it is considered that provision of a monochrometer for selecting only a certain energy spread from an energy distribution of an electron beam and for causing the selected electrons to hit a specimen is advantageous. In the prior art monochrometer, however, the monochrometer is inserted in the illumination electron lens system independent of the electron gun.
The following two methods are adopted for the monochrometer inserted in the illumination lens system in this way. In one method, electrons accelerated to a given accelerating voltage are directly passed through the monochrometer. In this case, if the accelerating voltage for the electron beam is increased, the dispersion power of the monochrometer deteriorates. Consequently, it is difficult to obtain an energy resolution of less than 0.2 eV.
In the second method, electrons accelerated up to a desired accelerating voltage are once decelerated and passed through a monochrometer to select only electrons with a desired energy spread. Then, the electrons are again accelerated. In this method, the energy of electrons passed through the monochrometer can be made low. Therefore, high energy dispersion power can be obtained. The energy spread of the electron beam passed through the monochrometer can be reduced to about 10 meV. In this method, however, electrons once accelerated are decelerated and reaccelerated. Where the accelerating voltage is in excess of 200 kV, multistage deceleration and multistage acceleration are necessary. This makes the instrument bulky.
In view of the foregoing circumstances, a third method is conceivable. That is, a monochrometer is mounted within an electron gun. Emitted electrons are passed through the monochrometer before being accelerated to a given accelerating voltage. This is a monochrometer within an electron gun.
In these second and third methods, however, a filter forming the monochrometer or a slit for selecting some energy of electrons must be placed at a high electric potential. Generally, the filter itself has no mechanical movable parts and so it is possible to place the filter at a high electric potential. However, it is difficult to place the slit, which must be mechanically movable, at a high electric potential.
In particular, it is customary to mechanically adjust the position and width of a slit. Therefore, where the slit is placed at a high electric potential, great problems will be produced. Especially, in the third method (i.e., in the case of a monochrometer within an electron gun), it is considered that mechanically adjusting the position and width of a slit at a high electric potential is substantially impossible.
Accordingly, it is an object of the present invention to provide a monochrometer for use with a slit that does not need to be moved. This can be especially preferably mounted within an electron gun.
To achieve the above-described object, the present invention provides a monochrometer which is mounted within an electron gun and to be used with an electron source for producing an electron beam, the monochrometer comprising: a dispersing filter for dispersing the electron beam according to energies of electrons; and an energy-selecting slit placed in a path of the electron beam dispersed by the dispersing filter. This energy-selecting slit is made of a single slit plate placed to block electrons having energies more than or less than a given energy. For example, the slit plate is a beam-blocking member made of a single metal plate having a straight edge.
In one feature of the invention, the electron source is a thermal emission type electron source, a Schottky emission-type electron source, or a tunneling field emission-type electron source.
In another feature of the invention, the position of the electron beam relative to the energy-selecting slit can be adjusted by controlling power supplies for driving the filter.
The present invention also provides a monochrometer inserted between a specimen and an electron source for producing an electron beam, the monochrometer comprising: a dispersing filter for dispersing the electron beam according to energies of electrons; and an energy-selecting slit placed in a path of the electron beam dispersed by the dispersing filter. This energy-selecting slit is made of a single slit plate placed to block electrons having energies more than or less than a given energy. For example, the slit plate is a beam-blocking member made of a single metal plate having a straight edge.
In a feature of this monochrometer, the electron source is a thermal emission-type electron source, a Schottky emission-type electron source, or a tunneling field emission-type electron source.
In another feature of this monochrometer, the position of the electron beam relative to the energy-selecting slit can be adjusted by controlling power supplies for driving the filter.
Other objects and features of the invention will appear in the course of the description thereof, which follows.